Plasma processing systems generate a plasma in a processing chamber that is then applied to a workpiece. The workpiece is supported in the chamber on a platen, a pedestal, a chuck, or some other support. The plasma may be used to perform a variety of different processes on the workpiece. These include etching material off the workpiece, doping materials that are already on the workpiece and adding material to the workpiece by deposition. The effects of these processes can be controlled by photolithography, screening, masking, coating and other processes.
The plasma itself is generated using a high voltage power supply and an input gas. The plasma generates a collection of ions and electrons with positive, negative and neutral electric charge that in combination has an overall quasi-neutral electric charge. This collection of plasma near the generator is driven across a sheath toward the workpiece by a bias voltage applied to the workpiece. The bias voltage generates an electrical field around the workpiece to attract ions through the sheath.
The plasma sheath is formed on any surface exposed to the plasma. There is a high electric field within the sheath. The field can accelerate positive ions and repel negative ions and electrons. When a workpiece is placed in a processing chamber and exposed to the plasma a sheath forms in a shape that conforms to the top of the workpiece surface. Similarly, a sheath will form on a grid placed between the plasma and the workpiece. The thickness and electric field of the sheath are determined by the relative voltage potential between the workpiece and the plasma. By applying a bias voltage to the workpiece, the ions of the plasma can be accelerated by the sheath to the workpiece.
In some systems, there is a grid between the sheath and the workpiece to direct the ions of the plasma sheath in a particular direction. The angular propagation of the ions can be used to control how the etching, doping, or deposition processes affect the workpiece. The ion energy distribution function (IEDF) describes the distribution of energy with which the ions impact a workpiece surface. The ion angular distribution function (IADF) describes the distribution of the angle of incidence of the ions on the surface with respect to the normal from the surface. Ion energy distribution and angle distribution are descriptors for the interaction between a plasma and topographical features on the workpiece.